Alloy melting and holding furnace

ABSTRACT

An induction furnace comprising a upper furnace vessel; an induction coil positioned below the upper furnace vessel; and a melt-containing vessel positioned inside the induction coil and communicably connected to the upper furnace vessel, wherein the positioning of the melt-containing vessel inside the induction coil defines a gap between an outside surface of the melt-containing vessel and an inside surface of the induction coil. A system for direct-chill casting comprising at least one an induction furnace; at least one in-line filter operable to remove impurities in molten metal; at least one gas source coupled to a feed port associated with the gas; and at least one device for solidifying metal by casting. A method of cooling an induction furnace comprising introducing a gas into a gap between an induction coil and a melt-containing vessel positioned inside the induction coil; and circulating the gas through the gap.

CROSS-REFERENCE TO RELATED APPLICATION

The application claims the benefit of the earlier filing date ofco-pending U.S. Provisional Patent Application No. 61/908,065, filedNov. 23, 2013 and incorporated herein by reference.

FIELD

Induction melting and holding furnaces, and more particularly to suchdevices useful in the processing of alloys such as aluminum lithium(“Al—Li”) alloys.

BACKGROUND

Al—Li alloys and certain other highly alloyed aluminum alloys havetraditionally been melted using induction melt-furnace technology,generally coreless or channel induction types. Due to the chemicalactivity of lithium in aluminum, standard furnaces, thedesigns-of-combustion gas fired furnaces, are not used. To melt theAl—Li alloys, indirect inductively generated heat is applied using aninduction furnace's electromagnetic field, where the metal in thefurnace couples with the magnetic field to generate heat. Corelessinduction furnaces typically have a continuous coil, usually copper,surrounding the circumference of the body of the furnace. A channelinduction furnace has the induction coil mounted externally to the mainbody of the furnace, and uses a pass-through method to transfer moltenmetal through a heating zone. Channel induction furnaces are generallylarger than coreless induction furnaces, and were developed because thecoreless induction furnaces have a practical size limitation. For bothabove types of induction furnaces, the heat energy developed via themagnetic field as well as from the molten metal itself requires theinduction coils to be liquid cooled using water or glycol or mixturesthereof. Water is generally used as the coolant but this creates asafety issue if a furnace lining failure occurs. The molten metal couldpenetrate the furnace lining and reach the cooling coil, and if themolten metal penetrates the cooling coil itself, an aluminum explosioncould result from aluminum to water contact. A number of publications,including the Guidelines for Handling Molten Aluminum published by TheAluminum Association (USA), discuss explosions and the requirement tokeep molten aluminum away from water. When melting and processing Al—Lialloys, the potential for catastrophic explosions with water are greaterthan for conventional (non-lithium containing) aluminum alloys. For thisreason, several furnace manufacturers offer furnace cooling systems thatuse coolant other than water for coil cooling, particularly halogenatedglycols.

Typical aluminum alloys use standard industrial refractories as workinglining for the induction furnace. These include mullite, alumina andsilica-based materials installed as cast-in-place linings, refractorybrick and mortar linings, and precast made-to-fit crucibles. Thesematerials are inserted into the furnace body, along with otherintermediate materials to separate the molten aluminum from contactingthe furnace induction coils. The lining material exposed to the moltenmetal is considered an expendable, and is periodically replaced asneeded. The backup materials (which do not normally come in directcontact with molten aluminum alloy) have generally longer life thanthose that come in direct contact, and the backup materials aregenerally not routinely replaced during a working lining replacement.

Al—Li alloys require special working lining refractories due to thechemical activity of lithium in aluminum. Magnesium oxide (MgO) andalumina (Al₂O₃) based refractories are typically used for corelessinduction furnaces, while silicon carbide (SiC) based refractories areused in the non-magnetic region away from inductor. For small,laboratory sized induction furnaces, SiC crucibles are used. The primarydrawback with MgO is its relatively low heat fatigue resistance. Thisnecessitates that the furnace be maintained hot and not drained onregular basis. This also poses a problem during alloy change as thefurnace generally cannot be cooled without cracking the refractory afterit has been used for melting metal. As a rule of thumb, if the MgOfurnace lining is allowed to cool below 1000° F. it will crack andbecome unusable. Because Aluminum melts at 1260° F. and is alloyed at1400° F., the lining has to be permanently kept at 1400° F. Thus,extraneous means are necessary to maintain heat in the furnace at alltimes, even when not in use, as well as between furnace operatingcycles.

Furnaces incorporating technologies other than induction have beenemployed for melting Al—Li alloys, including resistance heated vacuumfurnaces. Aluminum-lithium alloying processes have also used techniquesof post-furnace-in-line alloying of the lithium such that the lithiumdoes not contact or contaminate traditional furnace refractories; seeU.S. Pat. No. 4,248,630. Refractory products containing free silicaand/or phosphates are especially bad when used in conjunction with Al—Limelts, as the lithium preferentially attacks these materials, whichleads to almost immediate destruction of the ceramic.

U.S. Pat. No. 5,028,570 (“the '570 patent”) teaches thataluminum-lithium alloys that are used in aerospace applicationstypically contain about 2-3 percent lithium, which significantlyincreases the strength of aluminum and decreases the weight of the alloyrelative to pure aluminum. Only two refractories have been found thatcan provide a reasonable containment of these alloys. These areoxide-bonded magnesia and silicon-nitride bonded silicon carbide. The'570 patent describes silicon nitride bonded MgO, which is morecorrosion resistant to molten Al—Li. Additionally dry vibratory mixesconsisting of silicon carbide and alumina (manufactured and marketed byAllied Mineral Corporation, Columbus, Ohio and Saint Gobain Corp ofAmerica, Amherst, Mass.) are also employed in conventional corelessinduction furnaces used for melting aluminum lithium alloys. Pre-castand fired crucibles made out of tabular alumina (containing 96 percentof high purity tabular alumina, approximately 2 percent silica and 2percent titanium oxide) are also in use as main lining material of amelt-containing vessel in aluminum lithium applications. However, all ofthe above mentioned refractories react with aluminum lithium alloys andproduce alloys that tend to and develop spalling coupled with a networkof hairline cracks. The problem arises when during charging or skimmingor furnace wall cleaning—the refractory undergoes further mechanicalabuse. The mechanical abuse enhances the hairline cracks present in therefractory from thermal cycling. This, coupled with chemical reactionbetween the refractory lining and the lithium containing melt, andfurther the furnace filled with the low melting eutectics from the melt,gives rise to thicker sections of the entrapped semi-solid, solid,semi-liquid or fully liquid fins of the alloy to form a network withinthe refractory lining of the vessel wherein such network is slowlyprogressing to the outer wall of the lining. Because the inductiveenergy can readily couple with the network of aluminum or aluminum alloyfins of certain thickness (over 1.5 mm) trapped inside the lining, whenthe furnace is operated at a particular frequency and at required inputof electromagnetic power, the network of fins becomes superheated andrapidly advances to the outer boundary of the refractory lining. Theresulting failure of the refractory lining becomes a strong limitingfactor in the life of the furnace. If the failure of the refractorylining were to present itself only as a pure expense, it would onlyremain as an addressable cost item. However, the sudden advancement ofliquid aluminum lithium alloy towards the induction coil through thedamaged refractory lining of the melt-containing vessel presents acatastrophic explosion possibility if it were to reach even one or twoturns of the induction coil. Thus, absent a refractory material that ischemically inert to molten aluminum lithium alloys, there remains adistinct need to isolate the induction coil completely from therefractory lining of the melt-containing vessel.

Typical induction furnaces operate at very low electrical frequencies.To obtain stirring of the melt during the melting process, a lowfrequency is important to obtain a rapid melt rate. However, the rapidmelt rate makes the task of keeping the lithium in the melt moredifficult unless tightly controlled inert atmosphere is continuouslymaintained above the melt. U.S. Pat. No. 5,032,171 describes the use oflow frequency induction power to stir the melt vigorously such thatremoval of lithium is promoted. When using a higher frequency inductionfurnace, less stirring occurs, as movement of the molten metal is aninverse function of operating frequency. Higher frequency results inless stirring, however, higher frequency also results in coupling moreof the induction energy closer to the inside wall of the melt-containingvessel, and if the fins are present strong coupling and therebysuperheating of the fins results which additionally accelerates thedegradation of the refractory. Thus using lower frequency in the powersource cannot mitigate the degradation of the refractory lining. Anotherissue related to using low frequency (to achieve rapid melting) is theresultant forceful stirring that leads to the entrainment ofnon-metallic particles and undesirable oxides in the melt. Because lowerfrequencies result in more melt stirring, an operating frequencycompromise is often made to suit the operation but only at the expenseof doing more damage to the refractory lining and weakening the controlover the bath temperature.

For scrap melting, where quality is secondary to productivity, lowerfrequencies are typically used. When producing high quality melts,higher frequencies are used to reduce undesirable stirring, at theexpense of productivity.

Another fundamental factor connected with Al—Li melts is the degree ofhydrogen solubility in the molten Al—Li alloy. Because hydrogen iscompletely soluble in pure molten lithium (which melts at only 400° F.),the molten Al—Li alloy at 1400° F. captures significant amount ofhydrogen in the alloyed melt. For example, a furnace melt of typical nonlithium containing aerospace aluminum alloy AA 7050 will have hydrogencontent in a freshly prepared melt in a reverberatory melting furnace of0.5 cc/100 gms of molten alloy. As compared to this, the amount ofdissolved hydrogen in a freshly prepared melt of 1.2 percent Li alloymelted inside a controlled atmosphere induction furnace is 1.5 cc/ 100gms of molten alloy. Hydrogen in the regular aluminum alloys as well asin the aluminum lithium alloys is deleterious because it gives rise toporosity in the cast products. Such porosity in the cast condition ofthe alloy is difficult to heal during thermo-mechanical processing andaffects the strength, ductility, corrosion resistance and fatigueresistance of the finished products made from such castings that carryhigher amount of hydrogen. Besides the hydrogen coming in to the moltenAl—Li alloy through the addition of lithium, there is another sourcethat contributes to hydrogen pick up in the melt. This source ischemical in nature. Al—Li melts are extremely powerful reducing agentsand they strip away bonded hydrogen from components of the refractoryused in the melt-containing vessel. The binding agents used in thepreparation of the melt vessel refractory typically contain caustic orphosphoric acids or water or organic activators, all of which containsome amount of bonded hydrogen. This hydrogen can be stripped away by Aland Li atoms and is readily absorbed by the melt with simultaneousformation of Al—Li oxides, carbides, borides, etc. A representativechemical reaction is 2Al+3H20=Al2O3+6H, whereby large quantity ofhydrogen is liberated and retained by the melt.

Besides the above two contributors of hydrogen, there is yet anothersource of hydrogen transport in to the melt. This transport happensthrough the refractory of the melt-containing vessel of any standardinduction-melting furnace. The transport happens readily because a)there is higher partial pressure of hydrogen outside the outer wall ofthe refractory (which sits within the coil grout) than it is on theinside wall of the vessel refractory lining (which is in contact withthe melt), b) hydrogen being the smallest atom, the kinetics and thecoefficient of hydrogen transfer are very favorable to maintain acontinuous diffusion of hydrogen, driven by the hydrogen partialpressure difference. The coil grout is always in direct contact with theplant atmosphere and depending on the humidity (which is always high inan aluminum cast house since water is used as a heat extraction media),a reasonable amount of moisture (relative humidity 20 percent or higher)and thereby hydrogen is reminiscent on the outside surface of the coilgrout. To reduce such hydrogen pick-up in the melt transported throughthe refractory, the industry has found it necessary to employ anotherelectrical holding furnace to degas the specialty alloy melts includingAl—Li melts prior to casting. Such holding furnaces are of threedesigns, (i) vacuum is either applied on top of the Al—Li bath surface,or (ii) the exterior of melt-containing vessel is maintained in vacuum,or (iii) vacuum is applied at both locations, in the interior as well asthe exterior.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an embodiment of an inductionfurnace.

FIG. 2 is a schematic side view of a system operable to form one or morebillets or slabs or ingots from an alloy melt.

DETAILED DESCRIPTION

FIG. 1 shows a cross-sectional side view of an embodiment of aninduction furnace. In this embodiment, induction furnace 100 is atwo-part furnace with a bottom-located inductor. Induction furnace 100is capable of operating in a high and/or a low frequency mode rangingfrom 200 hertz to 80 hertz. Induction furnace 100, in this embodiment,includes upper furnace vessel 110, induction coil 120 positioned belowupper furnace vessel 110 (as viewed); and lower melt-containing vessel130 placed inside induction coil 120 and communicably connected to upperfurnace vessel 110. Identification of the inductor furnace as abottom-located induction type refers to the positioning or placement ofonly the lower or melt-containing vessel 130 inside induction coil 120rather than both melt-containing vessel 130 and upper furnace vessel110.

In one embodiment, melt-containing vessel 130 has a generallycylindrical shape with a representative with interior diameter of 10inches to 50 inches depending, for example, on a furnace melt raterequirement.

In the embodiment shown in FIG. 1, induction coil 120 is a coiledinduction coil defined by a coil or coils having lumen or opening 135therethrough through which a coolant such as a liquid coolant of wateror glycol or a gaseous coolant such as a refrigerant is introduced(e.g., pumped). In another embodiment, induction coil 120 may be a solidcore coil or an externally air cooled coil. In one embodiment, inductioncoil 120 has a generally cylindrical shape with having an interiordiameter that accommodates melt-containing vessel 130.

Illustrated in the embodiment of induction furnace 100 is gap 140between the outside surface 150 of the melt-containing vessel 130 andinside surface 160 of induction coil 120. Gap 140 is operable to allow agas to be circulated, entering from feed port 145 and exiting fromdischarge port 146 with feed port 145 and discharge port 146 associatedwith gap 140, respectively. In one embodiment, gap 140 is at leastone-half inch (0.5″), preferably 1.25 inches to 1.5 inches wide.Circulated in one embodiment means gas is introduced at feed port 145and moves within gap 140 around melt-containing vessel 130 and exits atdischarge port 146 to waste. In another embodiment, circulated means gasis introduced at feed port 145 and moves through gap 140 aroundmelt-containing vessel 130, exits at discharge port 146 and is thenreintroduced into feed port 145 (via a circulation loop). In eitherembodiment, it is desired that gas is circulated or moved around aportion, in one embodiment an entire portion, or substantially an entireportion of melt-containing vessel 130. In this manner, the gas isoperable to cool an exterior of melt-containing vessel 130. To aid inthe circulation of gas around melt-containing vessel 130, baffles may beadded that extend, for example, inside surface 160 of induction coil 120and direct the gas around outer surface 150 of melt-containing vessel130. The embodiment illustrated in FIG. 1 includes one feed port and oneexit port. In another embodiment, there may be more than one feed portand/or discharge port.

In one embodiment, the gas circulated through gap 140 is an inert gas.At least one inert gas selected from the group consisting of argon,helium, neon, krypton, xenon, and radon is circulated through the gapbetween the induction coil and the melt-containing vessel. Thecirculating gas has preferably at least 5 percent helium in it toimprove the heat transfer capability. In one embodiment, the circulatinggas comprises a mixture of about 80 percent argon and about 20 percenthelium. In another embodiment, the circulating gas is air. In yetanother embodiment, gas is air or nitrogen and an inert gas such ashelium. A representative circulation mechanism is run continuously solong as the furnace is at a temperature of 300° F. or over. Thecirculated gas exiting from discharge port 146 associated withmelt-containing vessel 130, in one embodiment, is cooled outside of thefurnace and re-circulated back into the gap (i.e., introduced into feedport 145 and gap 140). In one embodiment, a representative flow rate ofan inert gas is of the order of 12,000 cubic feet per minute (cfm) andthe temperature of the outer surface of the melt-containing vessel ismaintained below 150° F. This assures maintaining a freeze plane of themolten alloy well inside the refractory lining of the melt-containingvessel 130. In one embodiment, moisture from the circulated gas may beremoved before it is re-circulated with the use of an in-linedehumidifier. For certain aluminum alloys that do not contain reactiveelements such as lithium, the gas circulated through gap 140 can beatmospheric air input at ambient temperature and exhausted to theatmosphere. Reactive elements are elements that violently react withwater, hydrogen or a component of air (e.g., nitrogen, oxygen) at hightemperature. A representative flow rate of such air will be about 12,000cfm or as appropriate to keep the outside temperature of melt-containingvessel 130 at about 150° F. or lower.

The instantly described furnace vessel and method of circulating gasimprove the safety of melting and DC casting of Al—Li alloys byminimizing or eliminating ingredients that must be present for anexplosion to occur. It is understood that water (or water vapor orsteam) in the presence of the molten Al—Li alloy will produce hydrogengas. A representative chemical reaction equation is believed to be:

2LiAl+8H₂O→2LiOH+2Al(OH)₃+4H₂(g).

By maintaining a freeze plane within a melt-containing vessel 130, andpreferably within the vessel wall, well away from an outer portion ofthe vessel wall, the opportunity for molten Li—Al to escape from thevessel is inhibited. Such escape and contact with induction coil 120could otherwise be catastrophic.

In one embodiment, melt-containing vessel 130 has an exterior surfacethat is hoop-wrapped with tightly wound double tweed high temperaturefiberglass cloth cemented to an exterior of the containing vessel with asilicon carbide based high temperature refractory adhesives.Melt-containing vessel 130 is provided with a molten aluminum resistantworking lining that, in one embodiment, has an electrical resistivity ofbetween about 1,000 and about 10,000 micro ohm centimeters. In anotherembodiment, the resistivity is over 1,000,000 micro ohm centimeters. Inone embodiment, a working lining of melt-containing vessel 130 is arefractory ceramic.

To detect leak or bleed out of molten metal, at least one grid of micaconductor net is placed at or about outside surface 150 of themelt-containing vessel 130, the electrically conducting grid defined bythe net connected to a circuit to detect leakage of the melt. Suchcircuit may be linked to an alarm through, for example, a controller.Representatively, the mica grid is connected to an alarm system andworks as leak detection device by completing the electrical circuitbetween the metal and ground neutral when the leaked metal touches themica grid. In one embodiment, to assure further safety of operation,multiple grids of mica are placed in at least three locations including(i) the outer cylindrical surface of melt-containing vessel 130, (ii)bottom 142 of the melt-containing vessel 130; and (iii) at insidesurface 160 of induction coil 120.

For melt degassing purposes, a vacuum-generating device for degassing ofalloy melt in induction furnace 100 can be used. The vacuum-generatingdevice applies vacuum to a top surface of the alloy melt in inductionfurnace 100. Another method used for furnace degassing is to spargeargon gas using gas diffusor blocks of graphite or silicon carbide.

Upper furnace vessel 110 and melt-containing vessel 130 are communicablyconnected with interface ring 170 of, for example, silicon carbide andthermal ring-shaped gasket 180. The mating interface may be furthersealed with one or more rope gaskets 190 (e.g., titanium rope gaskets).

In the embodiment shown in FIG. 1, induction furnace 100 is of thetilting type, tilting along axis 192.

In one embodiment, clean out port is located at or near the upper end ofupper furnace vessel 110 and the steel shell. In one embodiment, it islocated opposite to the tilting axis. The shell has a refractory linedinterior for the containment of molten aluminum, including cover 195over the interior to seal the furnace atmosphere. Representatively, thefurnace atmosphere is maintained at argon pressure of one inch watercolumn (±0.75 inch water column). Representatively, an oxygenconcentration inside the furnace is 0.1 percent volume (from 0.05 to 0.2percent volume).

Furnace vessel 110, in one embodiment, includes a molten aluminum alloyresistant working lining; an intermediate layer of a high temperaturecompressible refractory material capable of allowing for expansion andcontraction of the working lining; and an outermost layer includingabout 70 percent alumina, about 10 percent silica, about three percentcalcium oxide and a binder material, all mounted inside a steel shellhaving typically one inch thickness, and wherein the inside diameter ofthe refractory lining provides for about 80 percent capacity of thetotal holding capacity of the furnace and the balance about 20 percentcapacity is taken up by the inductor and the region joining the inductorand the main vessel of the furnace. This proportion of capacities canalso be, respectively, about 90 percent and 10 percent.

In one embodiment, a working lining of furnace vessel 110 includesinnermost lining 112 of pure grade silicon carbide (SiC); nitride bondedsilicon carbide; yittria-stabilized zirconia with special additives forcontrolling chemical reactivity, or 85 percent SiC+15 percent alumina ortabular sintered alumina or high purity magnesia bonded with nitridebonded silicon carbide. This working lining is essentially devoid offree silicon, silicon dioxide, carbon fibers, graphite fibers, phosphatebonding gents, calcium aluminate, calcium silicate, cement, lime(calcium carbonate), on-crystalline weak oxides, amorphous weak oxides,or any other refractory, non-refractory, metallic additive or bondingagent that chemically reacts with molten aluminum alloys, andspecifically Al—Li alloys containing up to about 5 percent lithiumInnermost lining 112 has an inner surface coated with silicon carbidepaint 115 or plasma coated with zirconia, magnesium oxide or niobiummetal.

Referring to furnace vessel 110, vessel 110 includes back-up layer 116on innermost lining 112. In one embodiment, back-up layer 116 ismonolithically cast and sintered as a single unit. In anotherembodiment, back-up layer 116 is rendered of multiple isostaticallypressed and sintered building blocks assembled in tongue-and-grooveformation such as blocks 118 of innermost lining 112 and held togetherwith silicon carbide based high temperature mortar 119 or manufacturedas a hot isostatically pressed full size crucible, followed by hightemperature bake out and sintering.

Another back-up layer 117 on back-up layer 116 of furnace vessel 110, inone embodiment, is made of a high temperature compressible refractorymaterial capable of allowing for expansion and contraction of theinnermost lining 112 and back-up layer 116. Representative materials forback-up layer 117 include dried zirconia powder, a zirconia and aluminapowder mixture, and compressible thermally noon-conducting refractoryfibers which are non-wetting to molten aluminum alloys and is typicallytwo inches to four inches in thickness running on the interior of thesteel shell adjacent to ceramic paper 121, the ceramic paper beinginside the steel shell.

In one embodiment, induction furnace 100 is used to prepare melts ofLi—Al alloys which typically contain lithium in the range of 0.1 percentto 6.0 percent, copper in the range of 0.1 percent to 4.5 percent, andmagnesium in the range of 0.1 percent to 6 percent with silver,titanium, zirconium as minor additives along with traces of alkali andalkaline earth metals with balance aluminum. Such alloys are very easilyoxidizable in liquid state, react violently in liquid stage upon contactwith water but have much lower density (by 10 percent) than aluminumalloys and exhibit higher strength and stiffness. In another embodiment,induction furnace may be used to prepare melts of other alloys,including but not limited to, other aluminum alloys. In one embodiment,a Li—Al alloy prepared using induction furnace 100 has properties thatmeet the requirements of 100,000 pounds per square inch (psi) tensilestrength and 80,000 psi yield strength.

FIG. 2 presents a side view of a schematic of a system for forming oneor more billets or slabs or other forms in a direct chill castingprocess. According to FIG. 2, system 200 includes induction furnace 100as described in detail with reference to FIG. 1. Induction furnace 100includes furnace vessel 110 and melt-containing vessel 130 around whichan inductor coil is located (inductor coil 120, FIG. 1). In oneembodiment of making an Al—Li alloy, a solid charge of aluminum andlithium and any other metals for the desired alloy are introduced into alower portion of furnace vessel 110 and into melt-containing vessel 130.The metals are melted by induction heating and the melted metals aretransferred to first filter 210, through degasser 220, to second filter230 and to billet forming station 240.

Induction furnace 100 in system 200 includes an induction coil(induction coil 120, FIG. 1) surrounding melt-containing vessel 130. Asillustrated in FIG. 1, there is a gap (gap 140, FIG. 1) between anoutside surface of melt-containing vessel 130 and an inside surface(inside surface 160, FIG. 1) of the induction coil. In one embodiment,an inert gas is circulated in the gap. The representation of inductionfurnace 100 in FIG. 2 shows gas circulating around a representativelycylindrical melt-containing vessel (e.g., around the entire outersurface of the vessel). FIG. 2 shows a gas circulation subsystemassociated with system 200. In one embodiment, a gas, such as an inertgas, is supplied from gas source 255 through, for example, a stainlesssteel tube. Various valves control the supply of the gas. When a gas issupplied from gas source 255, valve 256 adjacent gas source 255 is openas is valve 251 to allow gas to be introduce into feed port 145 andvalve 252 to allow gas to be discharged from discharge port 146 into thecirculation subsystem. The gas is introduced into feed port 145associated with induction furnace 100. The introduced gas circulates inthe gap (gap 140) between melt-containing vessel 130 and the inductioncoil (induction coil 120, FIG. 1). The circulated gas then exitsinduction furnace 100 through discharge port 146. From discharge port146, the gas is passed through in-line hydrogen analyzer 258. Hydrogenanalyzer 258 measures an amount (e.g., a concentration) of hydrogen inthe gas stream. If the amount exceeds, for example, 0.1 percent byvolume, the gas is vented to the atmosphere through vent valve 259. Thecirculated gas from discharge port 146 is also passed through purifier260. Purifier 260 is operable or configured to remove hydrogen and/ormoisture from the inert gas. An example of a purifier to remove moistureis a dehumidifier. From purifier 260, the gas is exposed to heatexchanger 270. Heat exchanger 270 is configured to remove heat from thegas to regulate a gas temperature to, for example, below 120° F.Representatively, in circulating through the gap between the inductioncoil and the melt-containing vessel, a gas may pick up/retain heat and atemperature of the gas will rise. Heat exchanger 270 is configured toreduce the temperature of the gas and, in one embodiment, to return suchtemperature to a target temperature which is below 120° F. and, in oneembodiment, is around room temperature. In one embodiment, in additionto exposing the gas to heat exchanger 270, the gas may be cooled byexposing the gas to a refrigeration source 275. In this manner, thetemperature of the gas may be reduced significantly prior toentering/re-entering induction furnace 100. As shown in FIG. 2, the gascirculation subsystem 250 includes a temperature monitor 280 (e.g., athermocouple) prior to feed port 145. Temperature monitor 280 isoperable to measure a temperature of a gas being fed into feed port 145.The circulation of gas through the described stages of gas circulationsubsystem 250 (e.g., hydrogen analyzer 258, purifier 260, heat exchanger270 and refrigeration source 275) may be through a tube, e.g., astainless steel tube, to which each described stage is connected. Inaddition, it is appreciated that the order of the described stages mayvary.

In another embodiment, the gas circulated through the gap (gap 140,FIG. 1) between the melt-containing vessel 130 and the induction coil(induction coil 120, FIG. 1) is atmospheric air. Such an embodiment maybe used with alloys that do not contain reactive elements as describedabove. Referring to FIG. 2, where atmospheric air is to be introducedinto the gap, gas circulation subsystem 250 may be isolated to avoidcontamination. Accordingly, in one embodiment, valves 251, 252 and 256are closed. To allow the introduction of air into feed port 145, airfeed valve 253 is opened. To allow discharge from discharge port 146,air discharge valve 257 is opened. Air feed valve 253 and air dischargevalve 257 are closed when gas circulation subsystem 250 is used and agas is supplied from gas source 255. With air feed valve 253 and airdischarge valve 257 open, atmosphere air is supplied to the gap (gap140, FIG. 1) by blower 258 (e.g., a supply fan). Blower 258 creates anair flow that supplies air (e.g., through tubing) to feed valve 145 at avolume representatively on the order of 12,000 cfm. Air circulatesthrough the gap (gap 140), FIG. 1) and is discharged through dischargeport 146 to the atmosphere.

As noted above, from induction furnace 100, a melted alloy flows throughfilter 210 and filter 230. Each filter is designed to filter impuritiesfrom the melt. The melt also passes through in-line degasser 220. In oneembodiment, degasser 220 is configured to remove undesired gas species(e.g., hydrogen gas) from the melt. Following the filtering anddegassing of the melt, the melt may be introduced to billet- orslab-forming system 240 where one or more billets or slabs may be formedin, for example, a direct-chill casting process.

The system described above may be controlled by a controller. In oneembodiment controller 290 is configured to control the operation ofsystem 200. Accordingly, various units such as induction furnace 100;first filter 210; degasser 220; second filter 230; and billet formingsystem 240 are electrically connected to controller 290 either throughwires or wirelessly. In one embodiment, controller 290 containsmachine-readable program instructions as a form of non-transitory media.In one embodiment, the program instructions perform a method of meltinga charge in induction furnace 100 and delivering the melt to billet- orslab-forming system 240. With regard to melting the charge, the programinstructions include, for example, instructions for operating theinduction coil and circulating gas through the gap between the inductioncoil and melt-containing vessel 130. With regard to delivering the meltto billet- or slab-forming system 240, such instructions includeinstructions for establishing a flow of the melt from induction furnace100 through the fillers and degassers. At billet- or slab-forming system240, the instructions direct the formation of one or more billets. Withregard to forming one or more billets, the program instructions include,for example, instructions to lower the one or more casting cylinders 295and spraying coolant 297 to solidify the metal alloy cast.

In one embodiment, controller 290 also regulates and monitors thesystem. Such regulation and monitoring may be accomplished by a numberof sensors throughout the system that either send signals to controller290 or are queried by controller 290. For example, with reference toinduction furnace 100, such monitors may include one or more temperaturegauges/thermal couples associated with melt-containing vessel 130 and/orupper furnace vessel 110. Other monitors include temperature monitor 280associated with gas circulation subsystem 250 that provides thetemperature of a gas (e.g., inert gas) introduced into the gap (e.g.,gap 140, FIG. 1) between melt-containing vessel 130 and inside surfaceof the induction coil. By monitoring a temperature of the circulationgas, a freeze plane associated with melt-containing vessel 130 may bemaintained at a desired position. In one embodiment, a temperature of anexterior surface of melt-containing vessel may also be measured andmonitored by controller 290 by placing a thermocouple adjacent to theexterior surface of melt-containing vessel 130 (thermocouple 244).Another monitor associated with gas circulation subsystem 250 isassociated with hydrogen analyzer 258. When hydrogen analyzer 258detects an excess amount of hydrogen in the gas, a signal is sent to ordetected by controller 290 and controller 290 opens vent valve 259. Inone embodiment, controller 290 also controls the opening and closing ofvalves 251, 252 and 256 associated with gas circulation subsystem 250when gas is supplied from gas source 255 (each of the valves are open)with, for example, a flow rate of gas controlled by the extent to whichcontroller 290 opens the valves and, when ambient air is supplied fromblower 258, each of the valves are closed and air feed valve 253 and airdischarge valve 257 are open. In one embodiment, where air is circulatedthrough the gap (gap 140, FIG. 1), controller may regulate the velocityof blower 258 and/or the amount feed valve 253 is open to regulate atemperature of an exterior surface of melt-containing vessel 130 based,for example, on a temperature measurement from thermocouple 244 adjacentan exterior of melt-containing vessel 130. A further monitor includes,for example, probes associated with a bleed out detection subsystemassociated with induction furnace 100 (e.g., see mica probe discussionabove). With regard to the overall system 200, additional monitors maybe provided to, for example, monitor the system for a molten metal bleedout or run out.

The above-described system may be used to form billets or slabs or otherforms that may be used in various industries, including, but not limitedto, automotive, sports, aeronautical and aerospace industries. Theillustrated system shows a system for forming billets or slabs by adirect-chill casting process. Slabs or other than round or rectangularmay alternatively be formed in a similar system. The formed billets maybe used, for example, to extrude or forge desired components foraircraft, for automobiles or for any industry utilizing extruded metalparts. Similarly, slabs or other forms of castings may be used to formcomponents such as components for automotive, aeronautical or aerospaceindustries such as by rolling or forging.

The above-described system illustrates one induction furnace. In anotherembodiment, a system may include multiple induction furnaces and,representatively, multiple gas circulation subsystems including multiplesource gases, multiple fillers and degassers.

In the description above, for the purposes of explanation, numerousspecific requirements and several specific details have been set forthin order to provide a thorough understanding of the embodiments. It willbe apparent however, to one skilled in the art, that one or more otherembodiments may be practiced without some of these specific details. Theparticular embodiments described are not provided to limit the inventionbut to illustrate it. The scope of the invention is not to be determinedby the specific examples provided above but only by the claims below. Inother instances, well-known structures, devices, and operations havebeen shown in block diagram form or without detail in order to avoidobscuring the understanding of the description. Where consideredappropriate, reference numerals or terminal portions of referencenumerals have been repeated among the figures to indicate correspondingor analogous elements, which may optionally have similarcharacteristics.

It should also be appreciated that reference throughout thisspecification to “one embodiment”, “an embodiment”, “one or moreembodiments”, or “different embodiments”, for example, means that aparticular feature may be included in the practice of the invention.Similarly, it should be appreciated that in the description variousfeatures are sometimes grouped together in a single embodiment, figure,or description thereof for the purpose of streamlining the disclosureand aiding in the understanding of various inventive aspects. Thismethod of disclosure, however, is not to be interpreted as reflecting anintention that the invention requires more features than are expresslyrecited in each claim. Rather, as the following claims reflect,inventive aspects may lie in less than all features of a singledisclosed embodiment. Thus, the claims following the DetailedDescription are hereby expressly incorporated into this DetailedDescription, with each claim standing on its own as a separateembodiment of the invention.

What is claimed is:
 1. An induction furnace comprising: a upper furnacevessel; an induction coil positioned below the upper furnace vessel; anda melt-containing vessel positioned inside the induction coil andcommunicably connected to the upper furnace vessel, wherein thepositioning of the melt-containing vessel inside the induction coildefines a gap between an outside surface of the melt-containing vesseland an inside surface of the induction coil.
 2. The induction furnace ofclaim 1, further comprising at least one feed port and at least onedischarge port associated with the gap to the feed port and a gas sourcecoupled to the feed.
 3. The induction furnace of claim 2, wherein thegas source comprises at least one inert gas selected from the groupconsisting of argon, helium, neon, krypton, xenon, and radon iscirculated through the gap between the induction coil and themelt-containing vessel.
 4. The induction furnace of claim 2, wherein thegas sources comprises a gas mixture containing helium wherein heliumconcentration is at least 8 percent by volume.
 5. The induction furnaceof claim 2, wherein the gas source comprises a mixture of about 80percent argon and about 20 percent helium.
 6. The induction furnace ofclaim 2, wherein the gas source comprises air.
 7. The induction furnaceof claim 2, wherein a gas from the gas source is operable to becirculated between the feed port and the discharge port.
 8. Theinduction furnace of claim 2, wherein the circulated gas is cooledoutside of the furnace and re-circulated back into the gap.
 9. Theinduction furnace of claim 8, wherein the temperature of the circulatedgas in the gap is maintained below 150° F.
 10. The induction furnace ofclaim 8, further comprising a purifier, wherein the gas is configured tobe passed through the purifier before it is re-circulated.
 11. Theinduction furnace of claim 10, wherein the purifier comprises adehumidifier and the dehumidifier is operable to remove moisture in thegas to below 10 parts per million.
 12. The induction furnace of claim 8,further comprising a hydrogen analyzer operable to measure an amount ofhydrogen in the circulated gas and vent to vent valve the gas inresponse to a predetermined level of hydrogen.
 13. The induction furnaceof claim 1, wherein the gap is at least one-half inch.
 14. The inductionfurnace of claim 1, wherein the furnace is configured to tilt.
 15. Theinduction furnace of claim 1, wherein the induction coil is a cooledinduction coil.
 16. The induction furnace of claim 1, further comprisingat least one conducting grid of mica placed at or about the outsidesurface of the melt-containing vessel, said grid connected to a circuitto detect leakage of the melt.
 17. The induction furnace of claim 16,further comprising multiple grids of mica placed in at least threelocations comprising (i) the outer cylindrical surface of themelt-containing vessel; (ii) the bottom of the melt-containing vessel;and (iii) on the inner periphery of induction coil.
 18. The inductionfurnace of claim 1, further comprising a vacuum-generating devicecoupled to the upper furnace vessel and operable for degassing of alloymelt in the melt-containing vessel, said vacuum-generating deviceapplying a vacuum to a surface of the alloy melt in the furnace.
 19. Theinduction furnace of claim 1, wherein the upper furnace vesselcomprises: a molten aluminum resistant working lining of silicon carbidebased material; an intermediate layer of a high temperature refractory;and an outermost layer comprising about 70 percent alumina, about 10percent silica, about 3 percent calcium oxide and a binder material,allowing for expansion and contraction of the working lining.
 20. Theinduction furnace of claim 19, wherein the working lining has anelectrical resistivity of between about 1,000 and about 50,000,000 microohm centimeters.
 21. The induction furnace of claim 19, wherein theworking lining comprises pure grade silicon carbide (SiC); nitridebonded silicon carbide; yittria-stabilized zirconia with barium sulphateadditive for controlling chemical conductivity, or 85 percent SiC+15percent alumina or tabular sintered alumina or high purity magnesiabonded with nitride bonded silicon carbide
 22. The induction furnace ofclaim 19, wherein the intermediate layer comprises a material selectedfrom the group consisting of dried zirconia powder, a zirconia andalumina powder mixture.
 23. The induction furnace of claim 19, whereinthe outermost refractory layer of the upper case comprises about 70percent alumina, about 10 percent silica, about 3 percent calcium oxideand the balance binder material, and compressible thermallynon-conducting refractory fibers that are non-wetting to molten aluminumalloys.
 24. A system for direct-chill casting comprising: at least onean induction furnace comprising a upper furnace vessel; an inductioncoil placed below the upper furnace vessel; and a melt-containing vesselplaced inside the coil and communicably coupled to the upper furnacevessel, wherein the positioning of the melt-containing vessel inside theinduction coil defines a gap between an outside surface of themelt-containing vessel and an inside surface of the induction coil; atleast one in-line filter operable to remove impurities in molten metal;at least one gas source coupled to a feed port associated with the gas;and at least one device for solidifying metal by casting.
 25. The systemof claim 24, further comprising a leak-detection device to detect a leakof alloy melt from the furnace.
 26. The system of claim 24, furthercomprising a vacuum-generating device for degassing of the alloy melt.27. The system of claim 24, wherein the gas source comprises at leastone gas selected from the group consisting of argon, helium, neon,krypton, xenon, and radon.
 28. The system of claim 24, wherein the atleast one inert gas comprises a mixture of about 80 percent argon andabout 20 percent helium.
 29. The system of claim 24, wherein the atleast one gas source comprises air.
 30. The system of claim 24, whereina gas from the gas source is operable to be circulated in a circulationloop between the feed port and a discharge port associated with the gap.31. The system of claim 30, further comprising a cooling device coupledin the circulation loop.
 32. The system of claim 30, further comprisinga dehumidifier in the circulation loop gas.
 33. The system of claim 30,further comprising a hydrogen detector in the circulation loop.
 34. Thesystem of claim 24, further comprising a vacuum-generating devicecoupled to the upper furnace vessel and operable to apply a vacuum on asurface of alloy melt in the melt-containing vessel.
 35. The system ofclaim 24, wherein the gap is at least one-half inch.
 36. The system ofclaim 24, wherein the induction coil comprises a cooled induction coil.37. The system of claim 36, wherein glycol is used for cooling thecooled induction coil.
 38. A Li—Al alloy having properties that meet therequirements of 100,000 psi tensile strength and 80,000 psi yieldstrength and made using the furnace of claim
 1. 39. An aircraft partcomprising a product of an alloy made using the furnace of claim
 1. 40.An automotive part comprising a product of an alloy made using furnaceof claim
 1. 41. A method of cooling an induction furnace comprising:introducing a gas into a gap between an induction coil and amelt-containing vessel positioned inside the induction coil; andcirculating the gas through the gap.
 42. The method of claim 41, whereinthe gas is operable to cool a surface of the melt-containing vessel whena melt is contained within the melt-containing vessel.
 43. The method ofclaim 41, wherein circulating comprises introducing gas discharged froma discharge port associated with the gap into a feed port associatedwith the gap.
 44. The method of claim 43, wherein prior to introducingthe gas into the feed port, the method comprises reducing a temperatureof the gas.
 45. The method of claim 42, wherein the gas is air.
 46. Themethod of claim 45, wherein the melt comprises aluminum and at least oneother element that is not a reactive element.